This invention relates generally to means for detecting particulate matter and more specifically to an improved electrostatic small conductive fiber detector which can reliably sense particulate fibers 2 mm or less in length.
A prior art device consisting of a 1.5 inch diameter metallic sphere charged to a high DC potential is used to generate an electrostatic field such that whenever a fiber falls within the influence of the electrostatic field, it is converted into a dipole and is radially aligned and attracted to the spherical sensor. As the attracted fiber contacts the sphere, it is charged to a like potential and repelled. The charge removed from the sphere is sensed, amplified and shaped into a pulse whose magnitude is a function of fiber length. The pulse can then be stored in an appropriate memory device, and analyzed to give total fiber count per unit time, fiber length spectrum over a specified period, and various other functions relating fiber count, fiber length, and time as desired. This device is accurate and reliable in detecting and measuring fibers with lengths ranging from 3 mm to 12 mm.
Applications requiring accurate fiber count and length measurement of fibers 2 mm or less in length have been difficult if not impossible using the spherical sensor device of the prior art. This is due mainly to the inability to differentiate the small charge pulse transferred to a short fiber from electromagnetic interference and electronic background noise. An improvement in sensitivity of the prior art device was achieved by utilizing two spherical sensor units connected through a differential amplifier. The differential amplifier had high common mode rejection and thus eliminated interference common to both sensors allowing fibers with length as short as 1 mm to be detected. This method, however, required a duplication of sensors and associated equipment, and the addition of a high gain, common mode rejection differential amplifier to the circuit. Also, although it does give an improvement in sensitivity, the improved length detection spectrum is not sufficient for many purposes. Another means of increasing sensor sensitivity is decreasing the diameter of the sphere. This results in a larger charge being transferred to a fiber upon contact, but the decrease in spherical sensor diameter also results in a corresponding decrease in the effective capture area of the electrostatic field around the sensor. The reduction in capture area causes a corresponding reduction in fiber counts per unit time. Although the pulse size for a small fiber increases, the number of pulses caused by fibers decreases in relation to the number of similar amplitude pulses from electromagnetic and electronic interference and noise, which remain essentially constant, with a resultant decrease in the accuracy of detection.
It is therefore an object of this invention to provide an electrostatic particle detector that can detect, count and measure the length of small conductive fibers.
Another object of this invention is to provide an electrostatic small conductive particle detector using a minimum of components.
A further object of this invention is the provision of an electrostatic small conductive particle detector having increased sensitivity to small fibers while retaining a large capture area.
Still another object of this invention is to provide an electrostatic small conductive particle detector allowing sensor selection such that particle size sensitivity can be varied to detect a desired size range of particles while retaining a large capture area.
Yet another object of this invention is to provide an electrostatic small conductive particle detector whose sensing element can be formed into grids of large area while retaining small fiber sensitivity.
A still further object of this invention is to provide an electrostatic small conductive particle detector which can be used to detect particles entrained in a flowing gaseous medium, dispersed in a nonflowing gaseous medium, or dispersed in a vacuum.
Another object of the invention is to provide a detector which can determine concentrations of varying length conductive fiber particles per unit volume over a period of time.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.